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Reservoir
Limnology of
Arid Regions:
Problems with
Predictability.
David Walker Ph.D.
University of Arizona
Compared to North-
Temperate Regions.
Increased drainage area size.
Flashy hydrology.
Watersheds prone to increased
disturbance.
Elevation gradients.
Wildland Fire.
More productive with fewer bio-available
nutrients.
More Prone to the Effects of
Climate Change
Thinner snowpacks.
Relicts from last Pleistocene will dwindle.
Increased frequency and intensity of
erosive events.
Increased nutrient loading in aging
reservoirs.
From Meta- to Micro- Scale.
Phytoplankton assemblages are
extraordinarily patchy.
Overall biomass is difficult enough to
model!
Knowing what assemblage types to
expect under any given set of conditions
is extraordinarily difficult.
Large-Scale Models
Based on “mean-field” approaches.
Ecological patchiness is smoothed
Advantages:
Tractability
Few parameters
Clear interpretation
Disadvantages
Patchiness is smoothed out(!)
Micro-scale dynamics are impossible to
ascertain.
Yet these are often the metrics we are most
concerned about.
The effect of the multitude of possible
disturbance types is difficult to determine
Commonly Used
Lake/Reservoir Models for
TMDL Development
Watershed Models
Loading Simulation Program in C++ (LSPC)
Watershed Assessment Model (WAMview)
Storm Water Management Model (SWMM)
Receiving Water Models
A Dynamic One-Dimensional Model of
Hydrodynamics and Water Quality (EPDriv1)
Stream Water Quality Model (Qual2K)
Conservational Channel Evolution and
Pollutant Transport System (CONCEPTS)
Environmental Fluid Dynamics Code (EFDC)
Water Quality Analysis Simulation Program
(WASP)
EUTROMOD
BATHTUB
Problems with Quantification
“Examples of indicators for a nutrient TMDL
include total phosphorus concentration, total
nitrogen concentration, chlorophyll
concentration, algal biomass, and percent
macrophyte coverage.”
Target values for indicators then need to be
established.
“Although such discrete impaired and
unimpaired cutoffs do not exist in natural
systems, quantifiable goals nevertheless are a
necessary component of TMDLs.”
Square Pegs and Round Holes
Often nebulous correlations between
nutrient concentrations and both type and
amount of phytoplankton…or any indicator
for that matter.
We simply do not know all the environmental
requirements for any given species of alga to
grow and survive.
Let alone interactions between species.
Although models are able to predict pollutant concentrations and movement with decent accuracy, they often fail completely at determining the biological response.
Lake and Reservoir
Characterization is Difficult
All are unique
Spatial and temporal variability
Biotic interactions the we cannot (yet)
detect
Nebulous correlation between
environmental change, disturbance, etc.
and biotic response.
A Three-Pronged Approach
1) Coordinated monitoring and sampling.
Taking spatial and temporal variability into
account.
Covers the “uniqueness” of individual
areas.
Helps to understand the biotic response to
environmental conditions.
Needs to be on-going.
2) Field and Laboratory Studies
To determine specific lake/reservoir
responses following manipulation.
Replication and control.
Based upon logical findings and
observations.
INSERT CORING PIC
Constant Model Calibration
Data collected during monitoring and
field/laboratory studies used to determine
individual reservoir response to a wide
variety of environmental conditions.
Models individualized.
Model refinement should be on-going.
Heuristic.
A Narrative Approach
Square Pegs and Round Holes
We cannot quantify the, as of yet, un-
quantifiable.
Defining reservoir condition should include
a combination of quantitative, semi-
quantitative, and qualitative approaches.
A multi-tiered approach is the one most
grounded in reality.
Questions